When backlighting an LCD with cold-cathode fluorescent lamps, several diffusion films are often used in conjunction with prismatic films to try to bring more of the light toward the normal angle to the display to increase the brightness and contrast. However, using multiple films in this way increases the costs, cosmetic issues and thickness. In order to extract light from the waveguides in backlights, it is common to print diffusing white spots on the bottom of the waveguide; these spots reflectively diffuse light out of the waveguide. However, if no diffuser were used, these spots would undesirably leave a visible pattern. So, using a symmetric diffuser directly above the waveguide typically spreads light in all directions, and reduces the amount of light in the desired directions.
Some LCDs also employ a symmetric diffuser to spread the light out in both (i.e., x and y) directions in order to increase the viewing angle. These diffuser films, however, have their drawbacks; they do not adequately control the diffusion of light and result in decreased brightness, and usually send light in undesired, e.g., vertical, directions from an LCD, where viewers are not often looking. Asymmetric diffuser films can spread the light out in different directions, but these are typically present in LCDs as separate films or embossed or cast surface relief structures on films. Multiple films can add to the cost and complexity of the system and are undesirable in cost-conscious applications.
The increased optical efficiency of white and colored LEDs is making them a more appealing light source for LCD backlights. A significant difference between the CCFL and LED light sources is their coherence. CCFLs are closer to an extended source, while LEDs are more like a point source. As a result, the optical effect of speckle is more visible with LED sources than CCFLs. One method for reducing the appearance of speckle is to space diffusers apart. In a backlight, however, this typically increases the number of films, and light is scattered into undesirable angles, resulting in a decreased brightness.